1. Field of the Invention
This invention relates to a method of and apparatus for treating waste gas wherein waste gas containing noxious (gas) ingredients such as SO.sub.2 and NO.sub.x are irradiated with electron beams to change the noxious (gas) ingredients into the form of mist (sulfuric acid and/or nitric acid) or dust (ammonium sulfate and/or ammonium nitrate) and the resulting mist or dust is then captured with a dust collector or the like.
2. Discussion of the Background
One method of waste gas treatment whereby noxious gas) ingredients such as SO.sub.2 and NO.sub.x are removed from waste gases is a method wherein waste gas is irradiated with electron beams to form various active species such as O and OH radicals from oxygen, water, etc. in the waste gas so that the active species act on on the noxious (gas) ingredients in the waste gas such as to form mist and this mist is further changed into dust in the presence of ammonia or the like, the mist and dust then being captured with a dust collector or the like.
FIG. 1 schematically shows the structure of an electron beam irradiation portion of a waste gas treatment apparatus for carrying out this waste gas treatment method. As illustrated, a waste gas duct 1 is provided with irradiation windows 2, and waste gas passing through the waste gas duct 1 is directly irradiated with electron beams 4 emitted from electron beam accelerators 3 through the irradiation window 2. In the structure wherein the electron beams 4 are directly applied to the inside of the waste gas duct 1, when the amount of waste gas increases to that which would be treated in a practical application and the size of the waste gas duct 1 increases correspondingly, it has heretofore been necessary, in order to allow all of the waste gas to absorb the electron beams 4, to dispose a multiplicity of electron beam accelerators 3 (two in the illustrated example) at the outer periphery of the waste gas duct 1 and also to increase the maximum range of the electron beams 4, as shown by the chain lines 5. However, disposition of a multiplicity of electron beam accelerators 3 is disadvantageous in that the structure of the waste gas treating apparatus becomes complicated and cost is increased. Further, in order to increase the maximum range of the electron beams 4, it is necessary to raise the acceleration voltage for electron beams, which leads to a substantial rise in the cost of the electron beam accelerators 3. In addition, if electron beams are accelerated at high voltages, high-energy X-rays are generated and a thick lead or concrete wall or the like must be provided in order to provide shielding from such high-energy X-rays, which results in a rise in the overall cost of the waste gas treatment apparatus. Thus, the prior art suffers from various problems.
The following Table 1 shows the relationship between the maximum range of the electron beams according to the acceleration voltage therefor, the size of an electron beam irradiation chamber, the flow rate of waste gas and the thickness of a lead wall providing shielding from high-energy X-rays.
TABLE 1 __________________________________________________________________________ Maximum range Size of of electron electron Flow rate Thickness Accelerating beam in waste beam chamber of waste of lead voltage (KV) gas (m) (diameter, m) gas (m.sup.3 /h) wall (mm) __________________________________________________________________________ 1000 4.3 4.4 5 .times. 10.sup.5 257 800 3.1 3.2 2.5 .times. 10.sup.5 170 500 1.6 1.7 6 .times. 10.sup.4 90 300 0.8 0.9 2 .times. 10.sup.4 34 200 0.46 0.5 0.7 .times. 10.sup.4 13 __________________________________________________________________________
The above-mentioned Table 1 shows an example of the maximum range of electron beams and the size of the electron beam irradiation chamber which are necessary to allow the waste gas to absorb the electron beams, whereby the amount of waste gas capable of being treated and the thickness of the lead wall for shielding against X-rays, i.e. the size of the shielding structure, are determined. These are necessary conditions for the shielding against X-rays. In order to treat noxious gas ingredients such as SO.sub.2 and NO.sub.x, a suitable level of electron beam energy (accelerating voltage x amperage) is necessary. However, the amperage is not an important factor for determining the thickness of the lead wall for shielding against X-rays.
It should be noted that there are techniques which aim to have all of the waste gas irradiated with electron beams at a uniform dose, including those disclosed in the specifications of Japanese Patent Public Disclosure Nos. 49-096975 and 55-097232 and U.S. Pat. Nos. 4,507,265 and 4,596,642, but none of them completely solves the above-described problems.
Further, there is a technique disclosed in the specification of Japanese Patent Public Disclosure No. 61-68128 wherein atmospheric air is introduced into an electron beam irradiation reactor to allow the air to be irradiated with electron beams thereby to form ozone and oxygen atoms therein. The air having ozone and oxygen atoms is mixed with a waste gas to oxidize NO in the waste gas to form NO.sub.2 and then the waste gas is introduced to a wet absorption tower to effect desulfurization and denitration.
In the desulfurization and denitration of that technique, since a wet absorption tower is used the absorbing solution used in the wet absorption tower contains a large amount of nitrogen and sulfur compounds which are difficult to treat and costly waste water disposal equipment is therefore needed to treat the absorbing solution. This leads to the problem of high costs in the installation and maintenance thereof.